Polytetrafluoroethylene is a fluorocarbon which is widely used in shaft seals for sealing oil, grease, and other lubricants. It is usually advisable not to make any attempt to mold this material in the way in which most elastomers are molded in the shaft seal industry, but instead to use flat annular washers and to incorporate them in some manner into the shaft seal. It is known to form the washer into a partially frustoconical shape, and it is known to bond the washer to elastomer, which may itself be bonded to a metal case.
However, there have been problems in retaining true alignment of the polytetrafluoroethylene washer within the mold relative to the mold parts and the seal case during molding. Heretofore, the washer has normally been placed on a flat ledge with a portion overhanging a frustoconical portion of the mold, and then a ring of the uncured elastomer to be molded has generally been placed on top of it; upon mold closure the elastomer flows but at least a portion remains on top of the polytetrafluoroethylene washer. As a result, there is no direct contact between the upper mold member and the washer, so that there is no true positive retention of the washer in the mold; so it tends to slip out of alignment. The surface of the ledge on which it rests may be serrated, but since the upper mold member does not actually come against the washer and since it must transmit its pressure through a layer of elastomer, the retention is not always positive. As a result, the washer may be off center in the final molded product, or may in some other way lack the precision placement needed, all this simply because the washer was not positively controlled. Moreover, the positive control has not heretofore appeared to be feasible since the elastomer necessarily has to be molded and has to flow.
An object of the present invention is to assure positive retention of the fluorocarbon washer within the mold while the elastomer is being molded and bonded to it and while the fluorocarbon member is being shaped to have at least a partially frustoconical shape.
Another object of the invention is to provide a method in which the fluorocarbon member can be accurately retained on center during molding.
Another object of the invention is to provide a method and structure by which both sides of the fluorocarbon washer are directly engaged by mold members.
Another object is to provide that at least one of the mold members engaging the washer is provided with a surface which bites into the washer, and thereby assures its accurate retention.